Multi-level integrated circuit manufacturing requires many steps for metal and insulator film depositions followed by photoresist patterning and etching or other means of material removal. After photolithography and etching, the resulting wafer or substrate surface is non-planar and contains many features such as vias, lines, or channels. Often these features need to be filled with a specific material such as a metal, and then the wafer topographic surface needs to be planarized again, making it ready for the next level of processing.
Electrodeposition is a widely accepted technique for the deposition of a highly conductive material such as copper (Cu) into the features on the semiconductor wafer surface. Chemical mechanical polishing (CMP) is then employed to planarize the resulting surface.
In FIG. 1a, the large feature 1 and the small feature is are opened in the insulator layer 2, which is grown on a wafer. To fill these features with Cu, a barrier or adhesive layer 3 is first deposited over the whole wafer surface. Then a conductive Cu seed layer 4 is deposited over the barrier layer 3. Cu is electrodeposited over the whole surface by (1) making an electrical contact to the barrier layer 3 and/or the Cu seed layer 4; (2) placing the wafer in a standard Cu plating electrolyte; (3) placing an anode in the electrolyte; and (4) applying a negative voltage to the Cu seed layer with respect to the anode.
FIG. 1b shows the wafer after a short period of time which is adequate to deposit a Cu layer 5 with the thickness 5a. As shown in FIG. 1b, the Cu layer of nominal thickness 5a is adequate to fill in the small feature is since there is Cu film growth even on the conductive vertical walls of this feature. The large feature 1, however, is still not filled with Cu. To fill the large feature 1, Cu plating needs to proceed further, eventually yielding the structure depicted in FIG. 1c. 
As can be seen in FIG. 1c, in this conventional approach, the electrodeposited Cu layer 5 forms a very large metal overburden 6 on the top surface of the insulator 2 and over the small feature 1s. The overburden 6a over the large feature 1 is very small. The surface of the structure in FIG. 1c is non-planar, and therefore needs to be polished and planarized. The overburden and portions of the barrier layer 3 are customarily removed by CMP, yielding the structure in FIG. 1d, which has electrically isolated Cu-filled features. Removal of the large and non-uniform metal overburden of FIG. 1c from the wafer surface is time consuming and expensive and is a major source of dishing defects 6b in large features.
It would be highly desirable if the plating process could yield a Cu film which was planar and had a uniform overburden as depicted in FIG. 1e. CMP of such a substrate would be much faster and more economical and defects would be minimized. If the plating process could yield Cu-filled features with no overburden as depicted in FIG. 1f, then there would not be the need for CMP of the Cu layer. Only the portions of the barrier layer 3 on the top surface of the insulator 2 would have to be removed.
Electrodeposition is commonly performed in specially formulated plating solutions or electrolytes containing ionic species of Cu as well as additives that control the texture, morphology, and the plating behavior of the Cu layer. A proper electrical contact is made to the seed layer on the wafer surface, typically along its circumference, and the wafer surface is dipped in the plating solution. A consumable Cu anode or an inert anode plate is also placed in the electrolyte. Deposition of Cu on the wafer surface can then be initiated when a cathodic potential is applied to the wafer surface with respect to the anode (i.e., when a negative voltage is applied to the wafer surface with respect to the anode plate).
There are many Cu plating solution formulations, some of which are commercially available. One such formulation uses Cu-sulfate (CuSO4) as the copper source. James Kelly et al., J. Electrochemical Society, vol.146, p. 2540-45 (1999). A typical Cu-sulfate plating solution contains water; Cu-sulfate; sulfuric acid (H2SO4); a small amount of chloride ions; and a carrier, such as polyethylene glycols and/or polypropylene glycols. Some other chemicals are then added to this solution in small amounts to achieve certain properties of the Cu deposit. These additives can be classified under general categories such as levelers, brighteners, grain refiners, wetting agents, stress-reducing agents, and the like.
Commonly used levelers and brighteners are generally sulfur-containing compounds, such as derivatives of thiourea. Other levelers and brighteners are sulfonic acid derivatives, such as mercaptobenzene sulfonate. Other brighteners include 2,4-imidazolidine-diol, thiohydantoin, polyethers, polysulfides, and various dyes. There is a large volume of literature on the additives for Cu-plating solutions and their influence on the electroplated deposits. For example, U.S. Pat. No. 4,430,173 discloses an additive composition comprising the sodium salt of xcfx89-sulfo-n-propyl N,N-diethyldithiocarbamate and crystal violet, which shows excellent stability. U.S. Pat. No. 4,948,474 discloses a brightener additive for a Cu plating solution. U.S. Pat. No. 4,975,159 discloses lists of alkoxylated lactams and sulfur-containing compounds which were found to be effective additives. U.S. Pat. No. 3,328,273 describes Cu plating baths containing organic sulfide compounds.
Although a large volume of literature exists on the subject of additives to Cu plating solutions, many of the additive formulations are kept as trade secrets by plating solution suppliers. Some examples of Cu plating additive solutions provided commercially are: (1) CUBATH M(copyright) system, marketed by Enthone-OMI; (2) COPPER GLEAM(copyright) system, marketed by LeaRonal; and (3) ULTRAFILL(copyright) Addition agent and Suppressor, marketed by Shipley. Commercially available Cu plating solutions with additives typically yield bright and soft Cu deposits that have low stress. Copper layers deposited out of these solutions cannot be polished and planarized with the same solution, simply because the plating solutions are formulated only for plating, not for polishing or planarization.
Copper layers are traditionally polished and planarized by CMP in a machine specifically designed for polishing. In this method, the plated wafer is loaded onto a carrier head. The wafer surface covered with the non-planar Cu deposit (FIG. 1c) is brought into contact with a polishing pad and a polishing slurry. The polishing slurry contains oxidizing chemicals and micron or sub-micron size abrasive particles. When the pad and the wafer surfaces are pressed together and moved with respect to each other, polishing by the abrasive particles is initiated and the metal overburden is removed from the surface. A different CMP slurry is used to remove the barrier layer from the top surface of the insulator. The desired planar surface with electrically isolated Cu-filled features shown in FIG. 1d is eventually obtained.
The chemistry of the polishing slurry and the type of the abrasive particles used in a given CMP process are selected according to the chemical nature of the material to be removed. Therefore, the compositions of the polishing slurries for copper, tungsten, tantalum, tantalum nitride, silicon dioxide, and like materials that are used in integrated circuit (IC) manufacturing may all be different. For example, U.S. Pat. Nos. 4,954,142; 5,084,071; 5,354,490; 5,770,095; 5,773,364; 5,840,629; 5,858,813; 5,897,375; 5,922,091; and 5,954,997, all disclose various CMP slurry compositions for effective polishing of Cu. Slurries typically contain a solvent and a selection of abrasive particles, such as silica or alumina particles, which are suspended in the solvent. Furthermore, complexing agents such as NH3 and/or oxidizing agents such as NO3xe2x88x92 and Fe(CN)63xe2x88x92 are also included in the formulations to increase the dissolution rate of the abraded material and thus increase the Cu removal rate.
A typical CMP slurry has a high pH so that a passivating surface layer can be formed and sustained on Cu surfaces in the features where the pad cannot make high pressure contact. The surface layer over the Cu film protects Cu in such areas from chemical attack by the solution. High regions of the Cu layer making high pressure physical contact with the pad get polished because the abrasive particles can remove the passivating surface layer. The abraded material is then carried away from the wafer surface and dissolved by the slurry.
As discussed in J. Steigerwald et al., CMP of Microelectronic Materials, sections 7.2.1. and 7.2.2., John Wiley and Sons Inc. (1997), Cu is not expected to form a protective surface film in acidic solutions with low pH. Therefore, if an acidic slurry is employed for CMP, Cu in all regions, including in the recessed features, would dissolve into the acidic slurry and planarization as depicted in FIG. 1e would not be possible. In CMP of Microelectronic Materials, it is suggested that when using an acidic CMP slurry, a non-native, film-forming agent such as benzotriazole (BTA) may be added to the slurry composition to avoid chemical etching of the deposit in the recessed areas. However, the data in CMP of Microelectronic Materials also demonstrates that BTA may reduce the polishing rate.
According to CMP of Microelectronic Materials, during CMP, a protective layer such as an oxide layer first forms on the Cu surface due to the chemical composition and the pH of the slurry. The surface film is then efficiently removed by the mechanical action of the abrasive particles. Removed material is moved away from the vicinity of the wafer surface to avoid re-deposition. This process continues until all the metal on high surfaces making contact with the pad is removed. CMP slurries that are commercially available are all designed for polishing and planarization only. There is no CMP slurry formulation that allows plating of materials.
Thus, the chemical compositions of metal plating solutions and metal polishing and planarization slurries are very different. Polishing cannot be realized in prior art metal plating solutions, and plating cannot be achieved with standard CMP slurries. This is not customarily a problem since metal plating and CMP processes are carried out in different machines at different times, and the plating solutions and CMP slurries used in these machines are only expected to achieve their respective single functions, namely, deposition and polishing. However, this conventional approach is time consuming and it raises the manufacturing cost for integrated circuits.
It is an object of the present invention to provide a metal deposition solution that allows for polishing and planarizing the plated metal layer using the same solution. Metal layers deposited and planarized using such a solution would yield desirable flat surfaces as shown in FIG. 1e in a short period of time and would even achieve the structure depicted in FIG. 1f using a single machine. The idea of simultaneous plating and polishing or plating/polishing of a conducting material on a workpiece or wafer surface was disclosed in co-pending U.S. patent application Ser. No. 09/201,929, filed on Dec. 1, 1998, the entirety of which is incorporated herein by reference.
U.S. Pat. No. 6,004,880 discloses a modified CMP apparatus and technique to achieve simultaneous plating and polishing on a semiconductor wafer. However, compositions of solutions that can be successfully employed in such processes have not been fully disclosed. U.S. Pat. No. 6,004,880 discloses a modified CMP apparatus and a simultaneous plating/polishing technique that would be achieved by modifying a CMP slurry. In other words, the approach is modifying a CMP process to also do plating. Therefore, an electrolyte composition that might contain Cu-sulfate was proposed to be mixed into a CMP slurry. Such an approach of mixing plating electrolytes into polishing slurries has several drawbacks.
Electronic applications of electroplated metals, such as electroplated Cu, require not only planar surfaces, but also excellent electrical and mechanical properties. Copper films used in such applications should have low resistivity values close to the bulk resistivity of Cu, which is about 1.6xc3x9710xe2x88x926 ohm-cm. Films should also have good electromigration properties; should adhere well to their substrates; and should have large grains. Although planarization is important in the overall process, a highly planar Cu layer with high resistivity would not be usable in many integrated circuit applications where high performance is desired, because the limiting speed at which the circuit can be run is a strong function of the resistivity of the metal used in its structure. Copper is typically plated out of acidic electrolytes because layers obtained from these electrolytes have low resistivity and other desirable characteristics. Plating Cu out of a solution with large amounts of undesirable impurities typically deteriorate the properties of the deposited films. Therefore, the quality of Cu layers obtained from slurry/electrolyte mixtures would in all probability be poor.
Mixing plating solutions into CMP slurries to modify the slurries for plating would be also problematic for other reasons. The chemistry of plating solutions and polishing slurries are highly incompatible. The Cu-sulfate plating solutions that are commonly used for Cu plating are highly acidic solutions with pH values well below 0.5, typically below 0.1. High pH values deteriorate the plated film properties, typically giving rise to rough and/or burned deposits, especially at high plating current densities that are necessary for fast processing. This deterioration is because limiting current density values decrease as the pH of the solution goes up. In contrast, CMP slurries that are commonly used have pH values well above 2, typically above 4.0. For example, CMP slurries CPS-01 and CPS-03 sold by 3M(copyright) corporation have pH values of around 7. HASTILITE(copyright) marketed by Rhodes has a pH of 7.25. MICROPLANAR CMP 9000(copyright) by EKC Technology Inc. has a pH value of 8.83. As discussed in CMP of Microelectronic Materials, high pH is desirable in CMP slurries because it allows the formation of a protective surface layer on Cu. In acidic electrolytes with low pH values, surface layers such as Cu-oxides cannot be stable. It is therefore expected that mixing low pH plating solutions with high pH polishing slurries would have detrimental effects on both plating and polishing processes.
Slurries are formulated to keep their abrasive particles dispersed or in suspension. According to the basic theory of dispersions, dispersion characteristics of small particles in a solution such as a CMP slurry is a strong function of the pH of the solution. Changing the pH of a perfect dispersion may totally destroy that dispersion and cause the particles to agglomerate and precipitate out. Plating is also sensitive to pH changes and is affected by the electrolyte composition. Even parts per million (ppm) levels of impurities/additives in plating solutions have profound effects on the properties of the plated materials. Electronic applications of conductive materials such as Cu require deposition of good quality films with low resistivity and large grain. Chemicals in the polishing slurry would affect the quality of the plating solution and therefore the quality of the Cu layer that might be plated with an electrolyte/slurry mixture.
U.S. Pat. No. 6,004,880 discloses the idea of adding a plating solution to modify a polishing slurry for the simultaneous plating/polishing of Cu on a wafer with surface feature widths of preferably 1 micron or less. In many circuit designs, however, there are surface features with widely varying widths. The small feature in FIG. 1a, for example, may have a width of only 0.1-0.5 microns, whereas the width of the large feature may be 10-100 microns. The depth of the features may be in the range of 1-5 microns. A slurry/electrolyte mixture could be introduced onto a wafer surface with only submicron size features, if the particle size in the slurry is selected such that the particles are larger than the feature size. This way, the large particles cannot get into the wells or channels defined by the small features. However, if there are both large and small features on the wafer, then use of such a slurry would result in lodging of some of the abrasive particles into the large vias and channels. The lodged particles would then interfere with Cu plating into the features; increase resistivity; destroy microstructure and device performance; and drastically reduce the process yield.
Even if the plating solution and the CMP slurry could be mixed and used for plating and polishing, their recycling would be uneconomical. Once used, these solutions would have to be discarded and the cost of processing would be high.
The present invention solves these problems by modifying a plating solution for plating and planarization, rather than modifying a polishing slurry for polishing and plating as disclosed in the prior art.